feat(Mutual Checker): update "follows you!" icon

[AprilSylph]

Description

Updates the "follows you!" blog card icon to one of Tumblr's design system icons.

I believe this is the one usage of buildSvg that doesn't just use the same icon that's used for the feature in the control panel; removing this usage gets buildSvg closer to how I'm envisioning it would work in a world where we don't commit remixicon.symbol.svg.

This shouldn't change how the mutual icon renders at all.

Screenshots

Before	After	Before	After

Testing steps

1. Load the modified addon
2. Enable Mutual Checker
3. Find a post from a mutual
   • Expected result: The mutual icon is still inserted into the post header as normal
   • Expected result: Hovering the mutual icon shows the usual "Mutuals" hovertext
4. Hover a mutual's avatar
   • Expected result: The mutual icon is still inserted into the blog card as normal
   • Expected result: Hovering the mutual icon shows the usual "Mutuals" hovertext
5. Hover a follower's avatar
   • Expected result: The Tumblr following icon is inserted into the blog card
   • Expected result: Hovering the following icon shows the usual "{blogName} follows you!" hovertext

[marcustyphoon]

I think it will be possible to make an elegant system that does allow icons to be used that aren't a feature's primary icon, but this is an improvement anyway.

[marcustyphoon]

Okay, too messy to make an inline suggestion, but: the parentheses around the svg() calls seem unnecessary.

edit: Ah, eslint indents them in a weird way, I see.